interp: create interpreter interface value with new

In that case, the interface must be wrapped in an valueInterface
at creation.

With that fix, it is now possible to import
github.com/google/go-querystring/query. Not tested beyond that.

Fixes #1123.

interp/cfg.go

@@ -1324,7 +1324,7 @@ func (interp *Interpreter) cfg(root *node, sc *scope, importPath, pkgName string
 				// retry with the filename, in case ident is a package name.
 				sym, level, found = sc.lookup(filepath.Join(n.ident, baseName))
 				if !found {
-					err = n.cfgErrorf("undefined: %s %d", n.ident, n.index)
+					err = n.cfgErrorf("undefined: %s", n.ident)
 					break
 				}
 			}

interp/interp_eval_test.go

@@ -709,6 +709,29 @@ func TestEvalBinCall(t *testing.T) {
 	})
 }
 
+func TestEvalReflect(t *testing.T) {
+	i := interp.New(interp.Options{})
+	if err := i.Use(stdlib.Symbols); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := i.Eval(`
+		import (
+			"net/url"
+			"reflect"
+		)
+
+		type Encoder interface {
+			EncodeValues(key string, v *url.Values) error
+		}
+	`); err != nil {
+		t.Fatal(err)
+	}
+
+	runTests(t, i, []testCase{
+		{src: "reflect.TypeOf(new(Encoder)).Elem()", res: "interp.valueInterface"},
+	})
+}
+
 func TestEvalMissingSymbol(t *testing.T) {
 	defer func() {
 		r := recover()

interp/run.go

@@ -3303,11 +3303,20 @@ func _len(n *node) {
 
 func _new(n *node) {
 	next := getExec(n.tnext)
-	typ := n.child[1].typ.TypeOf()
+	t1 := n.child[1].typ
+	typ := t1.TypeOf()
 	dest := genValueOutput(n, reflect.PtrTo(typ))
 
+	if isInterfaceSrc(t1) && (!isEmptyInterface(t1) || len(t1.method) > 0) {
+		typ = zeroInterfaceValue().Type()
+	}
+
 	n.exec = func(f *frame) bltn {
-		dest(f).Set(reflect.New(typ))
+		v := reflect.New(typ)
+		if vi, ok := v.Interface().(*valueInterface); ok {
+			vi.node = n
+		}
+		dest(f).Set(v)
 		return next
 	}
 }